Hi Hans,
The expression should be evaluated for all grid rows so it should display properly on all rows. You are not doing anything strange in the expression implementation on the server so I'm currently completely in dark on what happens here.
First thing I would try is to replace "Function_Return (KENTEKEN.MERK * KENTEKEN.MODEL)" with "Function_Return Kenteken.Kenteken" or "Function_Return "TEST!"" to make sure there is nothing strange going on with the buffers.
Next step would be to look into the content of the AJAX calls that are send when the grid is filling itself. There might be several calls, you are looking for the call with "<sName>List_Fill_Find</sName>" in one of it first lines. The response XML will contain several "<m:TAjaxSnapShot>" items and they should all contain a field looking like:
Code:
<m:TAjaxField>
<m:sBinding>merkmodel</m:sBinding>
<m:sType>E</m:sType>
<m:sValue>TEST!</m:sValue>
<m:bChanged>false</m:bChanged>
</m:TAjaxField>
I've attached a screenshot of firebug that shows this call...